Clash=when two different ideas or anything comes together to cause a violent reaction. You can apply clash to many things and many situations. 

Colors/colours and clothes can clash also.

https://dictionary.cambridge.org/dictionary/english/clash

Crash = dictionary definition when something stops suddenly with catastrophic consequences.

When a person has a heart attack the hospitable calls for a "crash team" heart attack = suddenly stopping of heart consequences catastrophic

When planes, trains, boats ships, cars etc crash they stop suddenly consequences are catastrophic.

Many of the definitions for crash are not fully correct on the internet. It is not merely when two vehicles collide anything can crash as per the original definition of the word I gave you. that explains "crash teams" in hospitable. 

Computers crash. They suddenly stop working consequences are catastrophic or according to the error reports were is a "kernel panic" the core of the computer has meet an unknown situation and it goes into a preprogrammed  panic mode. 

Crush= to like some but not necessarily in a long term loving way. The kind of love teenagers are known for.

Crush also has another meaning to Crush something or someone to squeeze it/them causing harm or deformity.